Another try at explaining how the early Earth avoided a deep freeze

An unexpected greenhouse warming may have countered the faint young Sun.

Although life seems to have gotten a quick start on it, the early Earth was a nasty place. Very early on (and perhaps again later), meteors frequently peppered the surface. The atmosphere contained no oxygen, and consequently no ozone, to shield the surface from UV radiation. To top it off, the juvenile Sun burned about 25 percent less brightly than it does today, which should have put the average surface temperature below the freezing point of water.

Except it wasn't. Geological evidence indicates plentiful liquid oceans were present soon after the Earth’s formation, and the first one and a half billion years showed no signs of glaciation. This puzzle, dubbed the “faint young Sun paradox,” was recognized by Carl Sagan and George Mullen in 1972.

Various factors have been proposed to resolve the paradox. Some have suggested that the Earth was less reflective—due to a paucity of continental land area and decreased cloud cover—allowing it to absorb more incoming sunlight. Other answers have focused on greenhouse gases. A lot of effort has gone into investigating how much CO2 and methane were present, and researchers have proposed other, more exotic greenhouse gasses, like carbonyl sulfide (OCS) might have been present. The atmosphere was likely denser, which would enhance the effectiveness of greenhouse gases.

While combinations of these factors are potentially sufficient to solve the puzzle, plenty of questions remain. In a paper published in Science, Robin Wordsworth and Raymond Pierrehumbert of the University of Chicago have added another possible solution to the list—the interaction of hydrogen and nitrogen in the atmosphere.

Neither hydrogen (H2) nor nitrogen (N2) is technically a greenhouse gas. They can’t absorb infrared radiation emitted by the Earth—a fact determined by their molecular structure. However, some odd things happen when they collide with each other. For a moment, the two colliding molecules behave as one larger one, allowing the absorption of infrared energy. This process is known to be an important part of the energy balance on Saturn’s moon Titan.

Until recently, it was believed that the early Earth’s atmosphere would have contained very little hydrogen, which can escape into space due to its light mass. But it has since been demonstrated that the escape rate would probably have been slower than initially expected, and hydrogen may actually have been plentiful.

When the University of Chicago researchers went about calculating the effect this would have, they found that it could have warmed the Earth by as much as 10 to 15°C. If that was the case, moderate contributions from other factors might have been more than enough to keep the Earth out of the deep freeze. (Then again, another recent study suggests the paradox might be even bigger than previously thought, requiring a larger warming effect to resolve it. In that case, large contributions all around might still be needed.)

Obviously, this collision-induced absorption would only be active as long as the atmosphere had significant amounts of hydrogen in it. When methane-producing bacteria (methanogens) came on the scene, that would have started to change. Methanogens take in CO2 and hydrogen and release methane and water. Of course, methane is also a greenhouse gas, so the story here is complicated.

Because of the specific wavelength of infrared radiation that methane absorbs, its greenhouse impact is easily saturated at the concentrations relevant to the early Earth. Beyond that point, additional methane won’t make much difference. When methanogens arose, they could have quickly driven the atmosphere to that point, so the consumption of hydrogen and CO2 would then have made the net result a cooling one. (In addition, high methane concentrations lead to the formation of a reflective haze that causes some more cooling.)

The researchers suggest it’s possible that the widespread glaciation that occurred about 2.9 billion years ago could have counter-intuitively been caused by the biological production of methane. There are a lot of important unknowns in that scenario, however, and the authors note it’s “an important topic for future study.”

Writing in a perspective that accompanied the paper in Science, Penn State Professor James Kasting noted that the implications of the study extended beyond the Earth. “[T]he realization that H2 can warm terrestrial planet climates could be important for the prebiotic Earth, early Mars, and young Earth-like exoplanets. Large amounts of H2 in a planet’s atmosphere could allow it to remain habitable out to as far as [ten times the distance from the Earth to the Sun] around a star like the Sun,” he wrote. “[W]e will… need to keep this H2 greenhouse warming mechanism in mind as we decipher our own solar system’s history and search for other habitable planets like our own.”

22 Reader Comments

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

The Earth's tilt was also likely much less way back when the moon was much closer to the planet. Some researchers think at one point there was virtually no wobble so no changing of seasons or seasons lasted much longer than today. When I heard this it reminded me of the world in A Game Of Thrones where their seasons last years.

I'm no astrophysicist, but I do know that tidal heating (in which the tidal bulge of a planet causes internal friction which gives off heat) plays a large role in moons (e.g. Io, Europa) with eccentric orbits. We know that Io has geysers due to tidal heating and it is conjectured that Europa has liquid water under its global ice sheet. Intuitively, it seems like the moon (which was very very close to earth billions of years ago) would have exerted a very significant tidal force.

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

I think the heat flux is still much too small. The modeling paper linked in the 5th para from the end, for example, uses a mantle temp of 200 K warmer than today for their simulation.

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

The Earth's tilt was also likely much less way back when the moon was much closer to the planet. Some researchers think at one point there was virtually no wobble so no changing of seasons or seasons lasted much longer than today. When I heard this it reminded me of the world in A Game Of Thrones where their seasons last years.

Without axial tilt driving the seasons, orbital eccentricity would dominate. They'd still change at the same rate, but both hemispheres would have winter and summer at the same time. If eccentricity was the same as it is currently they'd be less intense; but (since there's a several percent chance of Mercury's orbit being perturbed to the point it and Venus's orbit overlap in the next few billion years) I don't think assuming the Earths orbit has remained fixed is a safe assumption. That said I'm not sure how we'd be able to determine what it's shape was billions of years ago.

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

The Earth's tilt was also likely much less way back when the moon was much closer to the planet. Some researchers think at one point there was virtually no wobble so no changing of seasons or seasons lasted much longer than today. When I heard this it reminded me of the world in A Game Of Thrones where their seasons last years.

Without axial tilt driving the seasons, orbital eccentricity would dominate. They'd still change at the same rate, but both hemispheres would have winter and summer at the same time. If eccentricity was the same as it is currently they'd be less intense; but (since there's a several percent chance of Mercury's orbit being perturbed to the point it and Venus's orbit overlap in the next few billion years) I don't think assuming the Earths orbit has remained fixed is a safe assumption. That said I'm not sure how we'd be able to determine what it's shape was billions of years ago.

Not according the the Science Channel's documentary on this subject. They had an expert on that said at one point there were no seasons like we have today. Wish i could remember the title of the show.

First, a necessary disclaimer: that CO2 GW is enough is an outstanding albeit minority alternative, which I like because of its simplicity. A nice review (which I haven't had time to read) is provided by the Loom, which claims: "It therefore remains to be seen whether carbon dioxide concentrations in agreement with geochemical evidence are sufficient to offset the faint young Sun."

The dominant position is espoused by Kasting, which has done the major heavy lifting in this area for decades. He lately switched from CO2 to CH4 as the dominant GW gas.

My position is influenced by many constraints.

- Due to AGW the knowledge of especially CO2 GW has increased recently. An oom less is needed than when kasting did his work. Only ~ 3 – 5 parts/thousand CO2 is needed, so there should be no conflict with the geological record.

“However, geological evidence seemed to indicate that the atmospheric CO2 concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing. With a radiative-convective model including new, updated thermal absorption coefficients, we found that the amount of CO2 necessary to obtain 273 K at the surface is reduced up to an order of magnitude compared to previous studies.”

“Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O2 concentrations and pH conditions in well-aerated soils have favoured the formation of ferric (Fe 3+)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H2 -rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO2 concentration was more than 100 times greater than today, causing the rain and ocean waters to be more acidic than today.

We therefore conclude that CO2 alone (without a signiﬁcant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.”

The review notes that these results have been “convincingly challenged”, but 1 of the 3 challengers is Kasting. I have read 2 of them IIRC, and I wasn't convinced. (But is no geochemist.)

- While the increase in knowledge on phylogeny goes exponentially, at least as of a few years ago methanogens were very derived. The very energetically demanding enzymes were seen to be derived from aerobic methylotrophic bacteria.

Eg methanogens most likely appeared during the onset of diversification that atmosphere oxygenation lead to, much too late to solve the earlier problem.

What about this new work then?

Well, it appears from Zimmer/The Loom that they suggest close to 30 % H2. An immediate problem with the Wordsworth-Pierrehumbert model is then that it takes the atmosphere towards hydrogen hydrodynamic escape at ~ 30 % H2, which removes massive amounts of all of it as other molecules are swept with IIRC. (Vague memories from an astrobiological course.)

I believe Kasting has the right attitude here. This mechanism may well stretch our current outer limits on habitability, set by methane dominated atmospheres on psychroplanets (think Mars) by a massive amount. In our case it may not be the solution.

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

I think the heat flux is still much too small. The modeling paper linked in the 5th para from the end, for example, uses a mantle temp of 200 K warmer than today for their simulation.

That is the current consensus AFAIK.

But since the tides could have been 1-2 order of magnitude larger than today, they could help prevent an icecover albedo lock in effect. Re your recent study of a larger problem: "Here we present the first comprehensive 3-dimensional simulations of the Archean climate that include processes as the sea-ice albedo feedback and the higher rotation rate of the Earth."

If there were little continent, and the sea-ice was broken up, it may not have been much of a problem. (The article is pay-walled, so I can't tell how they modeled the ice right now.)

Well, it appears from Zimmer/The Loom that they suggest close to 30 % H2. An immediate problem with the Wordsworth-Pierrehumbert model is then that it takes the atmosphere towards hydrogen hydrodynamic escape at ~ 30 % H2, which removes massive amounts of all of it as other molecules are swept with IIRC. (Vague memories from an astrobiological course.)

The Wordsworth-Pierrehumbert model actually used 10% H2 for these calculations, I believe. They mention in the intro that whichever paper argued for limited H2 escape (on the basis of lower incoming UV and adiabatic cooling limitations, I believe) said H2 could have been as great as ~30%.

Scott, tell me Ars has someone reviewing this ESD paper and will explain it soon, the anti-AGW crowd is already jubilating, and I don't understand a single word of the paper, so I don't know what to think of it.

Scott, tell me Ars has someone reviewing this ESD paper and will explain it soon, the anti-AGW crowd is already jubilating, and I don't understand a single word of the paper, so I don't know what to think of it.

Well, it appears from Zimmer/The Loom that they suggest close to 30 % H2. An immediate problem with the Wordsworth-Pierrehumbert model is then that it takes the atmosphere towards hydrogen hydrodynamic escape at ~ 30 % H2, which removes massive amounts of all of it as other molecules are swept with IIRC. (Vague memories from an astrobiological course.)

The Wordsworth-Pierrehumbert model actually used 10% H2 for these calculations, I believe. They mention in the intro that whichever paper argued for limited H2 escape (on the basis of lower incoming UV and adiabatic cooling limitations, I believe) said H2 could have been as great as ~30%.

I've long wondered about one other thing. Way back then, the Earth's cores, and mantle were much hotter than they are now. We all know how temperature rises as we go further down. It rises by a large amount. Several billion years ago, it's possible that this internal heat helped keep the surface warmer than otherwise.

It's thought that effects like this maintain a liquid ocean under the ice crust on Europa. I'm no geologist, but I don't see why the same effect couldn't have exsited on the early Earth.